This invention relates generally to charged particle beam milling and, in particular, to an apparatus and method for reducing differential sputter rates in crystalline and other materials.
Focused Ion Beam (FIB) microscope systems have been produced commercially since the mid 1980""s, and are now an integral part of rapidly bringing semiconductor devices to market. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the scanning electron microscope, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. In most commercial FIB systems, the ions used are positively charged gallium ions (Ga+) from liquid metal ion sources, however beams of other ions can be produced. For example, materials such as silicon, indium, cesium or even gases such as argon, krypton or oxygen can be utilized as ion sources.
Modem FIB systems can produce a beam of gallium ions as narrow as approximately 5 nm in diameter. One can increase the current of ions in the beam to operate the FIB as an xe2x80x9catomic scale milling machine,xe2x80x9d selectively removing materials wherever the beam is placed, and at the same time imaging the sample by correlating the known beam position with electrical signals produced as the incident beam interacts with the specimen. One skilled in the art would understand the operation of this well-known procedure.
Semiconductor devices such as microprocessors can be made up of millions of transistors, each interconnected by thin metallic lines branching over several levels and isolated electrically from each other by layers of dielectric materials. When a new semiconductor design is first produced in a semiconductor fabrication facility, hereinafter referred to as a xe2x80x9cfabxe2x80x9d, it is typical to find that the design does not operate exactly as expected. It is then necessary for the engineers who designed the device to test their design and xe2x80x9crewirexe2x80x9d it to achieve the desired functionality. Due to the complexity of building a semiconductor device in the fab, it typically takes weeks or months to have the re-designed device produced. Further, the changes implemented frequently do not solve the problem or expose a yet further difficulty in the design. Iterating through the process of testing, re-designing and re-fabrication can significantly lengthen the time to market of new semiconductor devices.
Over the past decade, techniques have been developed to allow FIB systems to reduce the time required for this procedure of perfecting a design. FIB instruments were first used to xe2x80x9ccutxe2x80x9d metal lines, typically comprised of alloys of aluminum and/or tungsten, on prototype devices, thus allowing for design verification in simple cases. Further, techniques have been developed using special gas chemistries in the FIB system to permit selective deposition of thin metallic lines to connect two or more conductors, selective removal of dielectric insulators but not metallic interconnects, and selective removal of metal interconnects without removing the dielectric insulators. Techniques have yet further been developed that allow the deposition of insulating materials. Hence, these advances in FIB system technologies now allow the cutting of metal interconnect lines, the insulating of these metal interconnect lines from their surroundings and the re-wiring of the lines to another location. Essentially, these capabilities now permit prototyping and design verification in a matter of days or hours rather than weeks or months as re-fabrication would require. This FIB xe2x80x9crapid prototypingxe2x80x9d is frequently referred to as xe2x80x9cFIB device modificationxe2x80x9d or xe2x80x9cmicrosurgery.xe2x80x9d Due to its speed and usefulness, FIB microsurgery has become crucial to achieving the rapid time-to-market targets required in the competitive semiconductor industry.
Until recently, the typical metals used for metallic interconnects were primarily alloys of aluminum and/or tungsten. The above described advances in FIB system techniques for cutting and depositing metal interconnects were specifically designed for these metal alloys and their particular physical characteristics.
Polycrystalline aluminum interconnect lines are composed of small, contiguous grains of aluminum. Within each grain, the atoms share a regular array-like order, but the relative position of the arrays of atoms can vary from grain to grain. This alignment of the arrays of atoms is known as the xe2x80x9ccrystallographic orientationxe2x80x9d of a given grain. Differences in crystallographic orientation can cause grains sputtered or milled with an ion beam to be removed at different rates, depending on the given orientation. For aluminium though, the difference between most of the slowest sputtering orientations compared to the fastest sputtering orientations is not particularly significant and hence is not a key factor in the techniques to cut and/or remove aluminum interconnects. Additionally, for aluminum, chemistries have been developed that selectively attack aluminum, causing grains of any orientation to sputter much more quickly in the presence of the gas than with just the ion beam alone. This process is well-known within the art and is commonly referred to as Gas Assisted Etching (GAE). In one particular well-known technique, chlorine gas is used to perform GAE of aluminum interconnects, cleanly removing aluminum grains, with little regard to their individual crystallographic orientation. The terms xe2x80x9cetchxe2x80x9d xe2x80x9cmillxe2x80x9d and xe2x80x9csputterxe2x80x9d are used interchangeably below.
Recently, copper-based interconnects have begun to replace aluminum-based interconnects in state-of-the-art devices due to the increased transmission speeds achievable with the use of copper. Unfortunately, FIB sputtering of copper is more difficult than sputtering aluminum alloys. Firstly, aluminum atoms have a lower atomic mass and less xe2x80x9cstopping powerxe2x80x9d than copper atoms, and simple ion beam milling of the copper atoms is less effective than the equivalent milling of aluminum. Further, a gas chemistry that permits GAE of copper has not yet been successfully developed. And yet further, the relative sputter rate between grains of different crystallographic orientations of copper can differ by a large factor, this factor being approximately 360% in some experimental tests.
Some of the difficulties that can occur when attempting to sputter copper interconnects that have different crystallographic orientations will now be described by example with reference to FIG. 1 and FIGS. 2A through 2I. FIG. 1 illustrates portions of three grains in a typical section of copper interconnect, sections 2, 3, and 4. The grains at each end section 2 and 4, in this example, have similar orientations, while the grain in the center section 3 is quite different. Consider a situation where the grains at each end section 2 and 4 are xe2x80x9cslow millingxe2x80x9d, whereas the grain in the center section 3 is xe2x80x9cfast milling.xe2x80x9d FIGS. 2A through 2I illustrate these three grain sections 2, 3, and 4 in the cross-section of a semiconductor device with two levels of copper interconnect. The three grain sections from FIG. 1 are represented in copper layer 5 in FIGS. 2A through 2I. As in a xe2x80x9crealxe2x80x9d device, they are covered with a protective dielectric material 6. They are also isolated, in the vertical dimension, from lower level conductors by a dielectric material 7. A second, lower layer of conductive interconnect is represented by a copper layer 8. Subsequent lower levels of the device containing transistors, etc. are not shown for clarity. Additionally, one skilled in the art would realize that copper layers 5 and 8 would extend to the left and right of the figure to carry electrical signals necessary to the functioning of the device and that the layers and grains are not shown to scale.
For purposes of illustration, FIGS. 2A through 2I show the case where it is necessary to perform microsurgery to modify the device by xe2x80x9ccuttingxe2x80x9d copper layer 5 (milling away all the conductive copper to produce an xe2x80x9copen circuitxe2x80x9d along this metal line) so as to verify a design change. FIGS. 2A and 2B illustrate the first steps, that is dielectric material 6 being removed by bombardment of ions 9 to expose copper layer 5. Dielectric materials tend to mill very uniformly in the FIB. In FIG. 2C, the incident Ga+ ion beam has cleanly and uniformly removed all of dielectric material 6, and begins to mill copper layer 5. As discussed above with reference to FIG. 1, copper layer 5 is composed of the three grain sections 2, 3, and 4 of copper, each end grain section 2 and 4 possessing a xe2x80x9cslowxe2x80x9d milling orientation, while the central grain section 3 has a xe2x80x9cfastxe2x80x9d milling orientation. FIG. 2D illustrates the result of the difference in milling speeds, that is the central xe2x80x9cfast millingxe2x80x9d grain section 3 is almost completely removed before even 25% of the thickness of the end grain sections 2 and 4 have been milled away. FIG. 2E illustrates the point where the central xe2x80x9cfastxe2x80x9d milling grain of copper layer 5 has been completely removed by FIB milling, while the xe2x80x9cslowxe2x80x9d milling grains at either end still retain more than 50% of their initial thickness.
At this point, one could assert that the initial requirement to xe2x80x9ccutxe2x80x9d copper layer 5 to provide an open circuit appears to have been accomplished. However, due to the three dimensional nature of the grains (not shown in FIGS. 2A through 2I, as the third dimension would be into the page), it is frequently necessary to proceed beyond this point to remove surrounding grains that would still be making connections (xe2x80x9cclosed circuitxe2x80x9d) in the third dimension of the metal line.
FIGS. 2F and 2G show the continuation of the milling process. As the central grain of copper layer 5 is now gone, dielectric material 7 begins to be milled away by the incident ion beam in the region below the now-removed xe2x80x9cfastxe2x80x9d milling central grain section 3 of copper layer 5. In a short time, this portion of dielectric material 7 is also milled away.
In FIG. 2H, the now exposed portion of copper layer 8 is now milled by the ion beam. If this portion also happens to contain a xe2x80x9cfast millingxe2x80x9d orientation of copper, it too is rapidly removed.
FIG. 2I shows the point where the last of copper layer 5 is finally milled away by the ion beam. Unfortunately, during this time, sufficient milling has occurred to the xe2x80x9cfastxe2x80x9d milling copper grains in copper layers 5 and 8 that the center of copper layer 8 has also been milled away, inadvertently cutting the interconnect line of copper layer 8, causing an unwanted open-circuit in this signal as well. To illustrate the effect of crystal orientation on milling rate, FIGS. 2A through 2I show that milling is uniform with a crystal grain although the milling is often non-uniform even within a single crystal grain of copper.
There are numerous other situations where this orientation induced difference in relative sputter rates in copper poses other difficulties. If FIB milling stopped at the point represented in FIG. 2G above, copper layer 8 is still intact, but now exposed. Subsequent deposition of conductive material in the FIB microsurgery process could lead to an unwanted short-circuit between the FIB deposited conductive layer and the now exposed copper layer 8.
Hence, there is a need for an improved technique to perform milling of copper, and more generally there is a need for an improved technique for milling elements that have large changes in milling rates based upon grain orientation. This improved milling technique will preferably lead to more uniform milling of such elements when being performed by an FIB.
The present invention comprises methods and apparatus for using a charged particle beam to uniformly remove material, particularly crystalline material, from an area of a target. The invention reduces differential sputter rates of crystalline structures and is particularly suited for FIB microsurgery of copper-based crystalline structures.
Uniformity of material removal can be improved by passing incoming ions through a layer formed on the surface of the material to be removed. Because the layer is typically removed during the milling process, it is referred to as a sacrificial layer. Uniformity of removal can also be improved by changing the morphology of the material to be removed, for example, by disrupting its crystal structure or by altering its topography.
In accordance with a preferred embodiment of the invention, a layer is formed over the material to be removed and an ion beam is directed toward the target. The layer interacts with the ion beam material removal process to increase the uniformity of the material removal by the ion beam.
The layer is preferably formed by the decomposition of a precursor gas in the presence of the ion beam. The deposition of the layer and the removal of the material are preferably a dynamic process, in which the layer is deposited by interaction of the precursor gas with the ion beam, the deposited layer then interacts with the ion beam to assist the removal of the material, and the layer itself is removed by the ion beam. This process preferably repeats at each dwell point of the ion beam until the material to be removed is completely removed.
According to one embodiment of the present invention, the method further includes applying a plurality of primer atoms prior to the applying of the sacrificial layer, the primer atoms enhancing the effectiveness of the sacrificial layer.
According to another embodiment of the present invention, the method further includes applying a plurality of de-conductive atoms that operate to diminish the conductivity of any residue left over from the sputtering operation. For instance, this de-conductive atom could comprise water vapour, oxygen molecules and/or other oxidizing agents.
According to another embodiment of the present invention, the method further includes milling the material in at least two steps, the first step resulting in an uneven morphology of the material and the final step resulting in a smooth, relatively planar floor at the milled area.
According to another embodiment of the present invention, the method further includes applying the ion beam in one or more steps at first dwell points or pixels spaced further apart than the beam diameter and then, to produce a smooth finish, at second dwell points spaced closer together than the beam diameter, a sacrificial layer preferably being applied while milling at the first dwell points and optionally, though not preferably, while milling at the second dwell points.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.